travis: just grab what we need from mbedtls

Author: tiennou

script/install-deps-linux.sh

@@ -3,9 +3,8 @@
 set -x
 
 if [ "$MBEDTLS" ]; then
-  git clone https://github.com/ARMmbed/mbedtls.git ./deps/mbedtls
+  git clone --depth 10 --single-branch --branch mbedtls-2.6.1 https://github.com/ARMmbed/mbedtls.git ./deps/mbedtls
   cd ./deps/mbedtls
-  git checkout mbedtls-2.6.1
   cmake -DENABLE_PROGRAMS=OFF -DENABLE_TESTING=OFF -DUSE_SHARED_MBEDTLS_LIBRARY=ON -DUSE_STATIC_MBEDTLS_LIBRARY=OFF .
   cmake --build .